Cutting opening in concrete joist floor system 1

[gte447f]

I have a project where a small hole or two (maybe 24"x24") need to be cut in a concrete slab. This is a one way concrete joist floor (pan joists) from the 1920's. I know these types of slabs are usually quite thin (maybe 2.5") and minimally reinforced with #3 bars because the spans are so short (usually only a couple of feet between joists). Provided that the holes can be positioned between joist webs so as to avoid cutting a joist, what issues need to be checked for the holes in the slab? Do the slab edges around the holes need to be reinforced? Will disrupting the continuity of the slab reduce the capacity of adjacent slab spans. Is it necessary to map the slab rebar prior to cutting to try to avoid cutting rebar? Thanks for any advice.

 

[ron9876]

Effective flange width for joists. Slab beam each side of openings with added load of half of the the width of openings.

If you don't know how the slab is constructed it may be best to add angles each side of openings spanning between joists.

 

[msquared48]

If the clear distance between the ribs is greater than 24", I would certainly consider mapping out the rebar to avoid cutting what I did not have to. Only adds to the integrity of the sgtructure.

One additional thought here... Since the concrete is from the 20's, the strength of the concrete is very likely much higher than originally specdified on the plans, assuming you are lucky enough to have any. Just keep that in mind.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Mike,
Why would you say that about the concrete strength? My experience with concrete from the 1920's is that it is highly variable, and in some cases only 500 psi or so.

 

[pmblair]

I agree with Ron9876, add angles to the sides of opening.

 

[JAE]

In the 1920's I think a lot of the joist systems perhaps didn't have much reinforcement in the slab - maybe some wire fabric.

If you cut an opening near the midspan of the slab, you might be affecting its flexural strength by removing some compression flange.

If you cut it near the end of the span, that is better for the joist but you might get near the compression flange of the supporting beam if there is one.

It may be that the original designer didn't use "T" beam calculations but you'd want to check it out anyway if you have the original drawings showing the joist reinforcement.

 

[BAretired]

There is a good chance that the clear distance between joists is less than 24" which would require cutting through a joist. Why not find that out before going any further?

BA

 

[gte447f]

Some good advice... thanks for contributing.

BA, the 24"x24" opening was just an estimate. The allowable opening size is up to me. The penetrations are for a group of 4" diameter electrical conduits. We are leaning toward grouping them and cutting one or two rectangular openings rather than coring a hole for each conduit. There are no drawings for the building. We will field verify the joist size, spacing, span, etc. before proceeding, but we haven't done that yet. If the joist are less than 24" apart, I will adjust the size of the opening.

Ron9867, you mentioned effective flange width of the joist. Are joists usually reliant on a compression flange? I thought that was one of the differentiating features between a joist and a T-beam. JAE also indicated that the original designer may have relied on a T-beam section. Without drawings, determining the design section may be difficult. Any thoughts? Also, is testing to determine the existing concrete compressive strength warranted?

Right now, the plan is to field verify the framing layout and map an entire bay (16.5'x16.5') for rebar locations. I am thinking the rebar locations may yield some insight into the original design intent, particularly if there is any slab rebar parallel to joist webs. I would like to place the opening near the end of the joists for the reasons stated by JAE, but I want to make sure that there isn't any negative moment rebar distributed uniformly within the slab at the girder line supporting the end of the joists. Other than that, I will probably reinforce the edge of the openings with steel angles spanning between joists as suggested by a couple of folks above.

 

[TXStructural]

Run quick calcs on the before and after at the proposed sections to see what the real impact will be. It is doubtful that there is longitudinal steel in the slab being used as compression steel, so run your calcs without considering it. Also, your calcs should consider actual loads, not "factored design loads".

You might be better off (structurally) coring rather than cutting one large opening, but it probably doesn't matter much. Cutting the slab reinforcement is likely unimportant (unless you find that the slab is required for flexure.)

Reinforcing the opening with short steel shapes will not improve joist strength unless is is attached sufficiently to act compositely and/or is at least as stiff as the concrete. You might consider adding a stiff diaphragm on each side of the opening between several joists to distribute loads/reduce deflection.

 

[msquared48]

Hokie:

Perhaps I assumed too much.

What I am saying is that if there are drawings, and if the concrete strength was specified in the drawings, assuming that the strength specified was originally achieved (and I realize that could not be the case), I have found that concrete inside buildings generally increases in strength over time. A core sample in the area of concern, and break, could determine that.

May not be the case, but could.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAE]

gte - older joist systems usually had top steel bundled up more in near the joist web and not spread out across the slab.

Yes, joists typically use T-beam design methods and count on the flange - at least in the last half of the 20th century. Not sure about the 1920's but I'd suspect they may have.

 

[TXStructural]

An example of issues that may complicate you assessment, from an older CRSI report:

Allowable Stresses. Some early authorities stated that allowable stresses in tension in the reinforcement higher that 12,000 psi show "very little to be gained in the economy" and recommended a maximum of 14,000 psi (Principles of Reinforced Concrete, Turneare, et al, 1908)). Recommended allowable stresses in tension in the 1924 Joint Committee Report ("Recommended Practice and Standard Specification for Concrete and Reinforced Concrete", ACI-AIA-AREA-ASCE-ASTM-PCA, 1924) were:
-16,000 psi for structural grade and rail-steel bars
-18,000 psi for intermediate and hard grade bars and twisted bars.

Much 1920's rebar did not have the same rib design, and bond and development was not as it is now (high bond ribs were standardized in the specs in 1947.)

Contact me directly if you need a copy of the CRSI note - it is under revision and is not available directly from the crsi.org website.

John Turner CSP PE
CRSI Greater Southwestern Regional Manager